In power transmission systems, faults like ground faults may occur. It is important to locate where such a fault occurs for instance in order to quickly remove it.
For long power lines, like overhead lines and cables, it is desirable to detect the location of a ground fault directly after the fault occurs in order to be able to dispatch repair crews to the correct position of the line or cable. Such power lines can be very long, for instance in range of 700-2500 km for overhead lines or in the range of 100-1000 km for submarine cables. Also combinations of overhead lines and cables are possible.
It is possible to determine the distance to a fault through analyzing a wave caused by the fault that reaches a measurement point. This is for instance described in the article “A Fault Locator for Three-Terminal Lines Based on Wavelet Transform Applied to Synchronized Current and Voltage Signals” by da Silva et al, 2006, IEEE PES Transmission and Distribution Conference and Exposition Latin America, Venezuela. The article describes the use of current detail signals of a wavelet transform for comparison with a self-adjustable threshold level. The current detail signals are harmonics of a basic Alternating Current (AC) signal. This type of distance to fault determination functions well in relation to AC power lines, but is less suitable for Direct Current (DC) power lines.
There is another way in which a wave can be analyzed and this is through using measurements at two different locations. The distances to a fault are here determined through measuring the instance in time a wave caused by a fault reaches two different locations and to determine the distance to the fault based on the time differences.
This principle is for instance described in U.S. Pat. No. 5,903,155, U.S. Pat. No. 6,822,457, EP 1,001,271 as well as in the article “A New Fault Location Method Avoiding Wave Speed and Based on Traveling Waves for EHV Transmission Line”, by Zhang Feng et al, 2008, THIRD INTERNATIONAL CONFERENCE ON ELECTRIC UTILITY DEREGULATION AND RESTRUCTURING AND POWER TECHNOLOGIES, VOL 1-6, APR. 6-9, 2008, 3rd International Conference on Electric Utility Deregulation, Restructuring and Power Technologies.
These types of fault distance measurements do function also for DC lines and are therefore good to use also in these situations.
In the above mentioned article by Zhang Feng et al the determination of the distance to a fault is also described as being based on detecting the moment when an initial wave surge arrives at a measurement point. This point in time is thus important.
There is one problem with this approach though and that is that the amplitude and shape of the fault wave that arrives at a measurement point can vary considerably based on where the fault occurs in relation to this measurement point. This has a negative influence on the accuracy of the measurement of the arrival time. It is therefore in some situations hard to determine the point in time that such a fault wave actually reaches a measurement point. Furthermore, as the distance to a fault is determined through a difference between two such arrival times, it is sufficient that one of the measurements is not precise enough in order to degrade the accuracy of the determination of the distance.
Also other factors influence the determination of the time a fault wave reaches a measurement point, such as varying operating conditions and varying noise levels.
There is therefore a need for providing a more precise determination of the time a fault wave reaches a measurement point of a power transmission system.